This invention relates to electronic field emission display devices, such as matrix-addressed monochrome and full color flat panel displays in which light is produced by using cold-cathode electron field emissions to excite cathodoluminescenct material. Such devices use electronic fields to induce electron emissions, as opposed to elevated temperatures or thermionic cathodes as used in cathode ray tubes.
Cathode ray tube (CRT) designs have been the predominant display technology, to date, for purposes such as home television and desktop computing applications. CRTs have drawbacks such as excessive bulk and weight, fragility, power and voltage requirements, electromagnetic emissions, the need for implosion and X-ray protection, analog device characteristics, and an unsupported vacuum envelope that limits screen size. However, for many applications, including the two just mentioned, CRTs have present advantages in terms of superior color resolution, contrast and brightness, wide viewing angles, fast response times, and low cost of manufacturing.
To address the inherent drawbacks of CRTs, such as lack of portability, alternative flat panel display design technologies have been developed. These include liquid crystal displays (LCDs), both passive and active matrix, electroluminescent displays (ELDs), plasma display panels (PDPs), and vacuum fluorescent displays (VFDs). While such flat panel displays have inherently superior packaging, the CRT still has optical characteristics that are superior to most observers. Each of these flat panel display technologies has its unique set of advantages and disadvantages, as will be briefly described.
The passive matrix liquid crystal display (PM-LCD) was one of the first commercially viable flat panel technologies, and is characterized by a low manufacturing cost and good x-y addressability. Essentially, the PM-LCD is a spatially addressable light filter that selectively polarizes light to provide a viewable image. The light source may be reflected ambient light, which results in low brightness and poor color control, or back lighting can be used, resulting in higher manufacturing costs, added bulk, and higher power consumption. PM-LCDs generally have comparatively slow response times, narrow viewing angles, a restricted dynamic range for color and gray scales, and sensitivity to pressure and ambient temperatures. Another issue is operating efficiency, given that at least half of the source light is generally lost in the basic polarization process, even before any filtering takes place. When back lighting is provided, the display continuously uses power at the maximum rate while the display is on.
Active matrix liquid crystal displays (AM-LCDs) are currently the technology of choice for portable computing applications. AM-LCDs are characterized by having one or more transistors at each of the display""s pixel locations to increase the dynamic range of color and gray scales at each addressable point, and to provide for faster response times and refresh rates. Otherwise, AM-LCDs generally have the same disadvantages as PM-LCDs. In addition, if any AM-LCD transistors fail, the associated display pixels become inoperative. Particularly in the case of larger high resolution AM-LCDs, yield problems contribute to a very high manufacturing cost.
AM-LCDs are currently in widespread use in laptop computers and camcorder and camera displays, not because of superior technology, but because alternative low cost, efficient and bright flat panel displays are not yet available. The back lighted color AM-LCD is only about 3 to 5% efficient. The real niche for LCDs lies in watches, calculators and reflective displays. It is by no means a low cost and efficient display when it comes to high brightness full color applications.
Electroluminescent displays (ELDs) differ from LCDs in that they are not light filters. Instead, they create light from the excitation of phosphor dots using an electric field typically provided in the form of an applied AC voltage. An ELD generally consists of a thin-film electroluminescent phosphor layer sandwiched between transparent dielectric layers and a matrix of row and column electrodes on a glass substrate. The voltage is applied across an addressed phosphor dot until the phosphor xe2x80x9cbreaks downxe2x80x9d electrically and becomes conductive. The resulting xe2x80x9chotxe2x80x9d electrons resulting from this breakdown current excite the phosphor into emitting light.
ELDs are well suited for military applications since they generally provide good brightness and contrast, a very wide viewing angle, and a low sensitivity to shock and ambient temperature variations. Drawbacks are that ELDs are highly capacitive, which limits response times and refresh rates, and that obtaining a high dynamic range in brightness and gray scales is fundamentally difficult. ELDs are also not very efficient, particularly in the blue light region, which requires rather high energy xe2x80x9chotxe2x80x9d electrons for light emissions. In an ELD, electron energies can be controlled only by controlling the current that flows after the phosphor is excited. A full color ELD having adequate brightness would require a tailoring of electron energy distributions to match the different phosphor excitation states that exist, which is a concept that remains to be demonstrated.
Plasma display panels (PDPs) create light through the excitation of a gaseous medium such as neon sandwiched between two plates patterned with conductors for x-y addressability. As with ELDs, the only way to control excitation energies is by controlling the current that flows after the excitation medium breakdown. DC as well as AC voltages can be used to drive the displays, although AC driven PDPs exhibit better properties. The emitted light can be viewed directly, as is the case with the red-orange PDP family. If significant UV is emitted, it can be used to excite phosphors for a full color display in which a phosphor pattern is applied to the surface of one of the encapsulating plates. Because there is nothing to upwardly limit the size of a PDP, the technology is seen as promising for large screen television or HDTV applications. Drawbacks are that the minimum pixel size is limited in a PDP, given the minimum volume requirement of gas needed for sufficient brightness, and that the spatial resolution is limited based on the pixels being three-dimensional and their light output being omnidirectional. A limited dynamic range and xe2x80x9ccross talkxe2x80x9d between neighboring pixels are associated issues.
Vacuum fluorescent displays (VFDs), like CRTs, use cathodoluminescence, vacuum phosphors, and thermionic cathodes. Unlike CRTs, to emit electrons a VFD cathode comprises a series of hot wires, in effect a virtual large area cathode, as opposed to the single electron gun used in a CRT. Emitted electrons can be accelerated through, or repelled from, a series of x and y addressable grids stacked one on top of the other to create a three dimensional addressing scheme. Character-based VFDs are very inexpensive and widely used in radios, microwave ovens, and automotive dashboard instrumentation. These displays typically use low voltage ZnO phosphors that have significant output and acceptable efficiency using 10 volt excitation.
A drawback to such VFDs is that low voltage phosphors are under development but do not currently exist to provide the spectrum required for a full color display. The color vacuum phosphors developed for the high-voltage CRT market are sulfur based. When electrons strike these sulfur based phosphors, a small quantity of the phosphor decomposes, shortening the phosphor lifetimes and creating sulfur bearing gases that can poison the thermionic cathodes used in a VFD. Further, the VFD thermionic cathodes generally have emission current densities that are not sufficient for use in high brightness flat panel displays with high voltage phosphors. Another and more general drawback is that the entire electron source must be left on all the time while the display is activated, resulting in low power efficiencies particularly in large area VFDs.
Against this background, field emission displays (FEDs) potentially offer great promise as an alternative flat panel technology, with advantages which would include low cost of manufacturing as well as the superior optical characteristics generally associated with the traditional CRT technology. Like CRTs, FEDs are phosphor based and rely on cathodoluminescence as a principle of operation. High voltage sulfur based phosphors can be used, as well as low voltage phosphors when they become available.
Unlike CRTs, FEDs rely on electric field or voltage induced, rather than temperature induced, emissions to excite the phosphors by electron bombardment. To produce these emissions, FEDs have generally used a multiplicity of x-y addressable cold cathode emitters. There are a variety of designs such as point emitters (also called cone, microtip or xe2x80x9cSpindtxe2x80x9d emitters), wedge emitters, thin film amorphic diamond emitters or thin film edge emitters, in which requisite electric field can be achieved at lower voltage levels.
Each FED emitter is typically a miniature electron gun of micron dimensions. When a sufficient voltage is applied between the emitter tip or edge and an adjacent extraction gate, electrons quantum mechanically tunnel out of the emitter. The emitters are biased as cathodes within the device and emitted electrons are then accelerated to bombard a phosphor generally applied to an anode surface. Generally, the anode is a transparent electrically conductive layer such as indium tin oxide (ITO) applied to the inside surface of a faceplate, as in a CRT, although other designs have been reported. For example, phosphors have been applied to an insulative substrate adjacent the gate electrodes which form apertures encircling microtip emitter points. Emitted electrons move upwardly through the apertures in an arc type path, over the gate electrodes and back downwardly to strike the adjacent phosphor areas.
FEDs are generally energy efficient since they are electrostatic devices that require no heat or energy when they are off. When they operate, nearly all of the emitted electron energy is dissipated on phosphor bombardment and the creation of emitted unfiltered visible light. Both the number of exciting electrons (the current) and the exciting electron energy (the voltage) can be independently adjusted for maximum power and light output efficiency. FEDs have the further advantage of a highly nonlinear current-voltage field emission characteristic, which permits direct x-y addressability without the need of a transistor at each pixel. Also, each pixel can be operated by its own array of FED emitters activated in parallel to minimize electronic noise and provide redundancy, so that if one emitter fails the pixel still operates satisfactorily. Another advantage of FED structures is their inherently low emitter capacitance, allowing for fast response times and refresh rates. Field emitter arrays are in effect, instantaneous response, high spatial resolution, x-y addressable, area-distributed electron sources unlike those in other flat panel display designs.
While the FED technology holds out many promises, existing designs are not without drawbacks. Present FED designs typically comprise a transparent glass face plate having its inside surface coated with a transparent conductive layer such as an ITO layer that serves as an anode. The anode layer is coated with a phosphor pattern much as within a CRT. An x-y electrically addressable matrix of cold cathode field emitters is generally spaced apart from the phosphors by a large number of minute spacer structures to maintain a uniform gap between the emitter points and the opposing phosphor surfaces. To reduce voltage requirements and allow for a viable mean free path for the emitted electrons, a gettered vacuum is generally provided and maintained within this phosphor/emitter spacing. Typical construction and operating voltages for such devices are on the order of about 100 to 200 xcexcm for the emitter to phosphor spacing, 10xe2x88x925 to 10xe2x88x927 Torr for the spacing vacuum environment, 500 to 1500 V for the cathode to anode voltages for high voltage and sulfur based phosphors (xcx9c100 V for low voltage phosphors), and 15 to 70 V for the cathode to emitter gate potentials.
Although lower operating voltages are preferred, particularly for portable applications, maximum luminous efficiencies are achieved at higher voltages particularly for the high voltage sulfur based phosphors. Because low voltage electrons do not have sufficient energy to penetrate the aluminum coating generally used behind the phosphor layer to reflect light toward the viewer in a CRT, FEDs typically use unaluminized phosphors. In addition light conversion efficiencies are generally higher in the 10 to 20 kV range used in traditional CRTs.
Use of higher voltage levels in the typical FED constructions gives rise to a special set of problems, however. Given the narrow emitter to phosphor gap and the presence of the spacers, there is a definite potential for electrical arcing especially along the spacer sidewalls. The problem is made worse when the spacers are contaminated by phosphor decomposition and sputtering resulting from normal operation of the device, particularly when the sulfur based phosphors are used.
It has been appreciated that it may be possible in theory to move to higher voltage levels by increasing the phosphor to emitter gap. It has been suggested this may require electron beam focusing, such as by fabricating an electrostatic lens over each pixel emitter matrix, to avoid the kind of pixel to pixel cross talk encountered with VFDs. Another issue is that larger gaps would generally require a higher vacuum, to maintain the mean free paths for the emitted electrons. Further, manufacturing feasibility issues are raised by the spacers, if the spacer heights are to be increased while maintaining the small spacer diameters required for the pixel densities in a high resolution display, or if large area displays are to be realized using the FED technology.
Still another issue with FEDs is the problem of cathode emitter poisoning that can result from decomposition of the phosphors, particularly the sulfur based phosphors, as previously described with respect to VFDs. The problem is only made worse by moving to higher voltage and hence electron energy levels which would tend to increase the decomposition rates of the bombarded phosphors.
While extensive research and development has been devoted to FEDs in recent years, the noted problems essentially remain unsolved. It was against this background that the present invention has been conceived.
It is accordingly an object of this invention to provide a low cost, high efficiency field emission display having the superior optical characteristics generally associated with the traditional CRT technology, in the form of a digital device with flat panel packaging.
Another object of the invention is to provide a field emission display device, for either monochrome or full color applications, with improved light conversion efficiencies, and with greater cathode to anode voltage level flexibility.
Another object of the invention is to lower the voltage requirements for high brightness cathodoluminescence within a field emission display device with improved light conversion efficiencies.
Another object of the invention is a field emission display device with improved light conversion efficiencies and a smaller emitter to phosphor gap within the device.
Another object of the invention is a field emission display device with improved light conversion efficiencies and a lower working vacuum within the device.
Another object of the invention is a field emission display device with improved light conversion efficiencies and in which requirements for an emitter to phosphor gap or an internal vacuum are either reduced, or altogether eliminated in the case of an all-film emitter/screen device structure.
Another object of the invention is a field emission display device in which improved light conversion efficiencies may be achieved without problems associated with pixel to pixel cross talk or need for special lenses to effect electron beam focusing.
Another object of the invention is a field emission display device in which plating of the anode materials or other materials on or into the phosphors is inhibited, to enhance the lifetime of the phosphors within the device.
Another object of the invention is a field emission display device in which decomposition or sputtering of the phosphors is inhibited, to thereby inhibit contamination of the emitters or any spacer structures within the device.
Another object of the invention is to provide a field emission display device with an improved mechanism for achieving gray scale resolutions within the device.
Still another object of the invention is to advance the use of gold-calcium as an electron emission amplification material, as well as the use of gold-calcium and other amplification materials for use within field emission display devices.
The invention applies generally to field emission display devices which use cathodoluminescence of a light emitting layer as a principle of operation. In such devices, a field emitter cathode matrix may be opposed by a phosphor-coated, transparent faceplate that serves as an anode and has a positive voltage relative to the emitter array matrix. The devices will typically incorporate a transparent conductive layer such as indium tin oxide (ITO) applied to the inside surface of the faceplate, or between the faceplate and the phosphor coating, to provide the anode electrode for applicable biasing with respect to the cathode-emitters. The phosphor coating may be masked or patterned on the faceplate to provide a matrix of x-y addressable pixels, with addressing provided via a selective cathode-emitter activation. The devices may use high voltage sulfur-based phosphors, or low voltage phosphors may also be used. Smooth deposited phosphor films on the order of about 1200 Angstroms thick are presently preferred for use with this invention, for improved light transmission.
In accordance with one aspect of the invention, the light emitting layer or pattern is electrically biased with respect to the anode, either with a DC or an AC potential, to generally lower the electron energy levels required for high-brightness, cathodoluminescent light emissions. AC biasing is presently preferred for high voltage phosphors (to discharge possible buildup of capacitive charges), and DC biasing is presently preferred for low voltage phosphors. Advantageously, the biasing potential can be adjusted or modulated to provide brightness or gray scale control within the display. A more general advantage is that phosphor biasing permits an FED to realize higher brightness levels. Also, smaller emitter-cathode to phosphor spacings and a lower vacuum than would otherwise be practicable can be used. For example, it may be feasible to use an emitter-cathode to phosphor spacing of less than 100 xcexcm, an internal working vacuum less than 10xe2x88x925 Torr, and an emitter-cathode to anode working potential less than about 500 volts (e.g., for high voltage phosphors), as may be desired. Preferably a biasing electrode will be in the form of a thin conductive film, disposed between the phosphors and the opposed cathode-emitters, applied either on the phosphors directly, or atop intervening film layers as will be described.
In accordance with a further aspect of the invention, amplification materials can be advantageously utilized to further lower the electron energy levels required for high-brightness, cathodoluminescent light emissions. Generally, a high-amplification-factor material layer can be disposed between the opposed cathode-emitters and the phosphors, applied either on the phosphors directly or atop intervening film layers, for producing secondary emissions of electrons when bombarded by primary emissions from the emitters. Preferably, the material used will have a high amplification factor on the order of that associated with copper-beryllium or silver-magnesium. Both of these materials have been used successfully for amplified secondary electron emissions in prior art photo-multiplier tubes and are well suited for use with this invention. Other suitable materials include copper-barium, gold-barium or tungsten-barium-gold, that are well known to have similar high amplification factors. Also, gold-calcium may be a particularly effective amplification material to use. An amplification layer thickness on the order of about 120 Angstroms is presently preferred, for effective amplification as well as transmission of primary emission energies and current for high-brightness display operations. Advantageously, the amplification layer can also serve as the biasing electrode, for purposes of phosphor biasing when implemented.
To achieve enhanced secondary electron emissions within the FED, an amplification layer can be applied over top of an amplification enhancement layer or film consisting essentially of an oxide of barium, beryllium, calcium, magnesium or strontium. Preferably, the amplification enhancement layer will be a near mono-molecular layer of magnesium oxide or beryllium oxide, itself applied either on the phosphors directly or atop intervening film layers.
To inhibit effects of phosphor sputtering or decomposition within the FED device, and to lessen or help eliminate requirements for emitter-cathode to phosphor spacings and a high working vacuum, a barrier layer in the form of a thin film of insulator material may be disposed between the emitter-cathodes and the phosphors. Preferably, this will be a thin silicon nitride layer applied directly on the phosphors, to permit the tunneling of electrons but inhibit the flow of ions or scattering of the phosphor materials within the device when the device is activated. A silicon nitride barrier layer thickness on the order of about 30 to 40 Angstroms is presently preferred. Other dielectric materials such as silicon dioxide, magnesium fluoride or polyamide materials (e.g., Kapton(trademark) polyamide film) may also be used for this thin film barrier layer.
To inhibit ion flow, migration or depositions of anode material on or into the phosphors, a thin film barrier layer of insulator material may be disposed between the anode and the phosphors to thereby enhance the phosphor lifetimes. A silicon nitride barrier layer with a thickness on the order of about 30 to 40 Angstroms is presently preferred for this purpose, to permit electron tunneling but inhibit anode to phosphor plating effects. Other dielectric materials such as silicon dioxide, magnesium fluoride or polyamide materials (e.g., Kapton(trademark) polyamide film) may also be used for this thin film barrier layer. A semiconductor material, such as amorphous or poly silicon, can also be used for this barrier layer.
The above-mentioned and other objects, features and advantages of the invention will become apparent from the further descriptions and the attached drawings.